The Obama administration said Sunday it was bringing in extra help to resolve some of the technical troubles that have hobbled the federally run health-insurance exchange, as it acknowledged the depth of the problems with the rollout of its signature domestic initiative.

The site "has not lived up to the expectations of the American people. We are committed to doing better," Health and Human Services officials said in a blog post. The post also said that "our team is bringing in some of the best and...